Oil, gas and other pipelines are typically formed from multiple lengths of individual steel pipe sections that are welded together end-to-end as they are being laid. As used herein, a section of pipeline is any length of a pipeline construction whilst a pipe section is what is welded together to form the pipeline construction. To prevent corrosion or other damage to the pipe sections occurring both from the environment and during transportation, and to reduce heat loss of fluids transported by pipelines, the pipe sections are coated with one or more protective and/or insulation layers. The pipe sections are usually externally coated at a factory remote from the location in which they are to be laid. This is often referred to as factory-applied coating and it is generally more cost effective than coating pipe sections on site where they are laid. At the factory, the coating is applied to the outside of the pipe sections whereupon a short length of approximately 150 mm to 250 mm is left uncoated at either end of the pipe section.
A coating may take several different forms depending on the particular coating applicator. A conventional coating will typically comprise at least a first, or ‘primer’, layer, such as a fusion bonded epoxy (FBE) material, that is applied in either liquid or powdered form to the outer surface of the steel pipe section while it is being heated. To ensure a good bond between the steel pipe section and the primer layer, the pipe section is typically blast cleaned and etched with an appropriate anchor pattern. The pipe section is heated by induction heating before the primer layer is applied. The desired temperature would normally be the curing temperature of the powdered or liquid primer material. On contact with the heated pipe section surface the primer material coalesces and cures to form a continuous layer. The primer layer mainly protects against corrosion. The primer layer may be used as the sole layer in a coating or it may be supplemented with a second layer to provide additional mechanical protective and thermal insulation properties.
Polypropylene, polyethylene, and polyurethane material have good mechanical protective and thermal insulation properties and they are commonly used to coat pipelines transporting fluids at up to 140 degrees Celsius. Polypropylene, polyethylene and polyurethane are widely used in factory-applied coating for pipe sections. While curing of the primer layer is ongoing, and so as to allow the layers to bond, a second layer of polypropylene, polyethylene or polyurethane coating is applied commonly by an injection moulding technique while the steel pipe section is heated by induction heating, for instance. All but the ends of the pipe section is enclosed by a heavy duty mould that defines a cavity around the uncoated pipe section, which is subsequently filled with molten polypropylene, polyethylene or polyurethane material from an injection moulding machine in the factory. Once the second layer has cooled and solidified, the mould is removed to leave the factory-applied coating in place on the pipe section.
Optionally, if polypropylene is used as the second layer in the coating, an additional layer of chemically modified polypropylene (CMPP) material which acts as an adhesive may be applied between the primer layer and second layer during the curing time (i.e. time taken to harden or set) of the primer layer. Likewise, if polyethylene is used as the second layer in the coating, an additional layer of polyethylene material which acts as an adhesive may be applied between the primer layer and second layer during the curing time of the primer layer.
Optionally, the second layer may comprise polypropylene or polyethylene material in the form of a tape wrapped in a helix over the first primer layer during the curing time of the primer. Optionally, the second layer may comprise a sleeve of polypropylene material heat-shrunk over the first primer layer during the curing time of the primer.
The uncoated ends are necessary to enable the pipe sections to be welded together to form a pipeline in the field. A section of pipeline where the ends of adjacent pipe sections are joined by welding is known as a field joint. After welding, the exposed ends of the steel pipe sections on either side of the weld (i.e. the field joint) must be coated. Field joint coatings may be applied using techniques similar, or equivalent, to the factory-applied coating techniques. The field joint coatings have, as far as is possible, the same mechanical and thermal properties as the factory-applied coatings by using compatible thermosetting plastics. Compatibility of the factory-applied and field joint coatings permits fusion to occur between the factory-applied and the field joint coatings, thereby imparting great integrity to the coatings at the field joint section of pipeline. To assist with fusion, exposed chamfers at the ends of the factory-applied coating on the pipe sections may be re-heated during the field joint operation.
Pipelines may be constructed in a dedicated facility where the pipeline is pulled through the facility in increments equal to the length of one pipe section, as is typical for offshore subsea pipelines. With this construction process each welding, heating and/or coating operation is performed in a fixed location with the field joint sections of pieline moving into the position where the operations will be performed. With this construction process it is not always necessary to lift the equipment onto or off the pipeline.
Pipelines may be constructed in situ, where the pipe sections are welded together and field-coated in, or very close to, the position in which the pipeline will be buried, as is typical for onshore cross-country pipelines. With this construction process the equipment must be transported to each individual field joint in order to perform a welding, heating and/or coating operation to that section of pipeline. The equipment is continually lifted on and off the pipeline in order to perform the operations sequentially along the chain of field joint sections of pipeline.
Aside from the differences caused by the need to continually lift equipment for field coating on and off the pipeline, the welding, heating and/or coating features are similar to equipment for use in a dedicated facility.
A known pipeline field joint coating applicator machine is disclosed, for example, in patent publication No. WO2009/024755. In this prior art publication there is disclosed a two-frame system for mounting on a pipeline whose field joints are to be coated with liquid or powdered coating material. An induction coil encircles the first cylindrical frame and is moveable axially along the pipeline in order that selected sections of pipeline (the field joints) may be heated to a temperature at which the coating may adhere to the surface of the pipeline. After heating, the first frame is moved axially so that the heated filed joint section of pipeline is then surrounded by the second cylindrical frame which carries a rotatable coating material applicator. Rotation of the applicator about the field joint applies coating material around the circumference of that section of pipeline.
Another known pipeline field joint coating applicator machine is disclosed, for example, in patent publication No. GB 2 181 396. In this prior art publication there is disclosed an apparatus for preheating and coating a section of pipeline comprising a cylindrical frame adapted to encircle and rotate about the section of pipeline in either direction, a pair of arcuate heating sections mounted on the frame in spaced circumferential relation with each other and a pair of single-nozzle coating applicators mounted in spaced circumferential relation to each other and between the heating sections. Each heater section comprises an array of water-cooled tubes, each tube being arranged in a flat coil sandwiched between parallel plates to form a so-called ‘flat pack’. The flat packs are separate elongate, longitudinally-orientated induction heaters spaced around the section of pipeline above the surface thereof adjacent to, and parallel with, the surface of the pipeline. The coating applicators are for coating material on the surface of the section of pipeline. The cylindrical frame is rotatable around the section of pipe while simultaneously applying an alternating electric current to the induction heaters to heat the section of pipeline to an application temperature for the coating material. The coating material is applied thereafter to the pre-heated section of pipeline through the material applicator while the cylindrical frame continues to rotate. The frame comprises an upper yoke section and two side yoke sections which pivot with respect to the upper yoke section, and locking means for locking the two side yoke sections together at their bottoms to compete a closure of the cylindrical frame about the pipeline.
A variety of equipment is available to coat sections of pipeline, largely aimed at reducing the time required to perform a coating process and economy of coating material, but also to help ensure a consistent application of coating material. For example, laying a pipeline typically involves coating several thousand field joints thus, even a small time saving in the time, or a small reduction in amount of coating material, required to coat each field joint can lead to significant overall cost savings. Likewise, consistent application of coating material can lead to significant improvements in coating quality and longevity.